Electric switch devices of the movable contact type are, in most cases, used in exacting conditions because such switch devices employ elements which are mechanically movable and which are accordingly subject to wear, abrasion or impact resulting from the sliding or striking motions of the movable elements. In the case of limit switches which are broadly used for the automatic control of various types of power-driven machines or equipment such as machine tools and industrial machinery for example, the movable or slidable elements incorporated in a switch device include not only the movable electric contacts but some slidable members of the switch housing and the elements mechanically connecting the slidable members to the movable contacts so that the switch assembly is subjected to more exacting conditions during use thereof because the sliding movement of the members are usually accompanied by generation of heat resulting from the friction between the slidable members. If, furthermore, a movable-contact switch device is of the oil-filled type having the contact elements enclosed within a sealed contact casing immersed in oil, repeated attacks of heat and mechanical impact to the switch device cause early deterioration of the sealing elements provided between the contact casing of the switch device and some of the slidable members forming part of the switch housing. When the sealing elements or some of the sealing elements thus isolating the contact elements from the oil in the switch housing are thus destroyed or otherwise damaged, the sealing elements fail to function properly and will permit entry of oil past the sealing elements into the interior of the contact casing from the oil chamber in the switch housing. The oil admitted into the contact casing tends to be burned and carbonized by the sparks which are produced between the movable and stationary contact elements within the contact casing when the contact elements are brought into or out of contact with each other. The fine particles of carbon thus produced around each of the contact elements deposit on each contact element. As the accumulation of the carbon particles in the contact casing proceeds, the deposits of the carbon particles on the stationary contact elements form an electrically conductive path between the individual stationary contact elements and destroy or at least impair the function of the switch device. If the switch device thus disabled is one of a number of similar switch units which are in operation concurrently, efforts for trouble shooting must be made for one after another of the individual switch devices until the particular switch device in trouble is found out. If it is desired to further inspect the switch device for the degree to which the contact elements are contaminated with carbon, the switch device must be wholly disassembled to measure the electrical resistance between the individual stationary contact elements of the switch device. Disproportionate amounts of time and labor are required of the efforts which must be paid for these purposes. The present invention contemplates elimination of these drawbacks which have thus far been encountered in conventional electric switch devices of the movable-contact type.